IntroductionTechnical professionals understand a variety of fluid-transferperformance concepts. The principles have much to do withevaluating if an individual pump, on an individual/microscale, will succeed in accomplishing its fluid-transfer dutieswith a reasonable degree of dependability. This includesevaluation of inlet/discharge conditions, flow, speed andpower requirements, as well as durability.This article explores a segment of the positive displacement(PD) pump arena where precise flow control is needed froma rotary-style PD pump (Figure 1). Despite the PD style ofoperation for these pumps, their use in precise meteringapplications has to be approached with caution becauseof the potential for excessive slip, which induces errors.Historically, instead of rotary pumps, reciprocating-typepumps have been favored in these types of applications.However, some processes cannot accept reciprocating-typepumps because of their inherent pulsation, cost, automationcomplexity, or other parameters. The NPSHr (net positivesuction head requirements) and stuffing needs ofreciprocating pumps are also a challenge.The ChallengeFigure 1 outlines the learn concepts to evaluate thesuitability of rotary positive displacement pumps forapplications needing precise flow control with variableprocess conditions, while factoring in pump wear.The application of advanced fluid-transfer concepts on amacro (or process) scale will enable entire processes tobecome efficient in addition to aiding the efficiencies of anyspecific pump. Users can find ways to produce a product atthe least cost considering factors such as plant-wide labor, floor space, capital investment, cleaning infrastructure andtotal process energy usage (Figure 2).For instance, users could replace batch-blending processeswith continuous in-line blending processes. New pumpswith good metering/predictable flow performance areenabling this process method switch.In its simplest form, a batch process (Figure 3) first involvessending ingredients in the correct amounts to a processingtank. Subsequently, and possibly in a distinct step, theproducts are mixed within the tank to produce the desiredblended product. In contrast, with an in-line continuous-blendprocess (Figure 4), the ingredients are fed in proportionallycorrect amounts and instantly combined as they aretransferred within a common manifold. This manifoldmay also contain in-line mixing devices to make surethe ingredients are properly blended.A full analysis of the benefits and drawbacks of trulycontinuous over batch processes are not possible in thescope of this article. In summary, however, continuousbatchprocesses can yield:■ Large reductions in floor space (no multi-stageblend tanks needed)■ Possible quicker product-formulation changes tomatch needs■ Reduced cleaning surfaces (eliminatingmulti-stage tanks)■ Capability of high degree of automation(recipe control)■ Reduced product losses and waste treatmentSeveral drawbacks in the use of continuous-blend processeshave been caused by limitations in the pumping technologyemployed. Past systems and some existing systems can beeffective, but cannot accommodate wide changes in processparameters like flow rates (affecting proportion limits) andviscosity (ingredient flexibility). Additional issues withexisting continuous in-line blending processes includestability as a result of startup/shutdown conditions,equipment aging and process upsets.New pump technologies, as well as correct selection ofexisting technologies, are now enabling the wider use ofcontinuous-blending processes that require more flexibilityand stability.Details on Pump PerformanceA pump’s “performance band” is the family of duty points(pump speed versus delivered flow rate) resulting frompump slip for a range of possible process conditions,including viscosity, back pressure, temperature and evenpump wear during its lifetime. The pump performance bandcan be described as either tight or loose, which indicateshow much the flow can change (think of slack) for a fixedpump speed. The performance band can also be describedas wide or narrow to indicate the possible range of speedsthe pump can run while producing flow.From a practical standpoint for in-line blending applications,the tighter the pump-performance band, the better themetering accuracy under varying process conditions. At thesame time, the wider the performance/flow rate band, themore flexibility in handling formulations that require awide range of possible ingredient input flows.This article explores these new concepts that take pumpperformance to the next level. In addition to the pump justsimply working, the correct application of these pumpconcepts allows refinement of the transfer process, permittingnew, enhanced applications that were previously not possibleor reliable. The in-line blending process described above isone example. Other examples include coating, spray drying,filling, filtering and heat-exchange processes that requirecontrolled flow with tight pump performance bands.

Tight Versus Loose Pump PerformanceThe root issue with rotary PD pumps is that the flowperformance on all pumps is to some degree affected byinternal clearances that result in slip. The degree of slipchanges with:■ Viscosity changes■ Differential pressure changes■ Clearance allowances for temperature change■ Wear (resulting in an increase in clearance)Given these product/process variables, tight performanceoccurs when the pump maintains close to its theoreticaldisplacement independent of changes to the abovevariables. The definition of a PD pump is a pump thattransfers a set displacement per unit operation, such asrevolution or stroke.Tight versus loose pump performance is the extent towhich, under a given range of conditions, the pumpmaintains high volumetric efficiency. High volumetricefficiency is the extent (ratio) in which the truedisplacement of the pump approximates its theoreticaldisplacement for given process/product conditions. Pumpslip is the difference between the theoretical displacementand the actual displacement. Therefore, the lower the pumpslip in any condition, the tighter the pump’s performanceunder conditions of changing viscosity, pressure, temperatureor wear.Classifying a pump as simply positive displacementwithout quantifying the tightness of its performance bandcan greatly affect the desired results in an application. Theextreme example is one in which, regardless of the pumpspeed, the slip is 100%. That is, all fluid that is pumpedforward then flows (slips) back through the pump’s internalclearances to produce no net fluid transfer. While soundingdramatic, it is not uncommon that a pump reaches thispoint (total loss of flow) before it is taken out of service tobe repaired or replaced.To understand slip for traditional PD pumps, see Figure 5. Itillustrates the possible loose-performance range (the yellowarea) of a typical PD pump when operating in variableconditions (changes in viscosity, back pressure, temperature,and wear). This graph shows how flow for a given pumpspeed (A) can vary from the theoretical (intersection BA) toan extreme (intersection EA) which indicates no flow. Thiscondition occurs in pumps with worn pumping elements,for example.Even in non-extreme cases such as when needing a flowrate of (B), the pump would need to be accelerated from(A) to (F) in order to achieve the flow (B). This can proveto be an automation challenge and result in a reductionof reliability. If the automation system does not have a wayto compensate for loss of flow and the pump remains at thesame speed (A), the flow rate (D) would be inadequate. Anactual curve for such a pump with 0.153 gallons/revolutionscan be seen in Figure 6.

Most users specifying pumps realize this and attempt tocontrol the extreme variabilities of viscosity, pressure,temperature and wear simultaneously.In many applications, this variation is sufficient to producea challenging operational scenario. In some cases, advancedautomation can help, such as using flow meters with speed/pressure control loops and back pressure stabilizationvalves. However, there are cases for which the possiblevariation cannot be compensated without recalibration orretuning the processes. These methods can prove costly orunfeasible, and could also increase system complexity (thusreducing reliability).Figure 5 illustrates a tight performance band, which isshown as the green performance band range superimposedon the same graph. Even with large variations in pumpingconditions within its published performance limits, themaximum variation in flow versus pump speed would bebetween (B) and (C) instead of (B) and (E), illustrated bythe yellow loose-performance band. An actual curve bandfor such a pump can be seen in Figure 7. Both pumps(Figures 6 and 7) have a theoretical displacement of 0.15gallons/revolution, but the curve in Figure 6 shows howloose the pump’s performance is at 250 rpm, producing asmuch as 28 gallons/minute of slip while attempting topump 38 gpm. The pump shown in Figure 7 has only 4gpm of slip under the same conditions.Today’s advanced pump manufacturers provide the tools thatpermit evaluating the possible slip for a given application.Curves are supplied that demonstrate how to down-rate theflow given changes in back pressure, viscosity or change ofinternal component clearance to handle certain temperatureranges. These tools are helpful for compensating for theperformance. At times, however, these performance changescan’t be adequately or reliably compensated and may notproduce optimal control.The Effects of Pump Component WearTo further complicate matters, pump component wearinvalidates most pump-performance curves. In highlyvariable conditions, wear cannot be accurately modeled orpredicted. For processes that require tight and predictableperformance over time, the solution is pumps that havetight performance ratios to begin with and are eitherimmune to wear or can compensate for wear. Pumps canalso be repaired to like-new condition. In doing this, therestill remains the risk that the pump’s performance willdegrade before the anticipated rebuild point and causeproduction issues. Repair or replacement to regain properpump performance can result in high costs for rotary PDpumps. In other words, the pump works mechanically justfine, but needs to be repaired to regain performance, whichcan be costly.Loose pump performance also has associated side effects.These include an increased amount of shear that isimparted on the fluid, greater power requirements (andreduced efficiencies) of the pump and heat generationNarrow Versus Wide Performance BandThis is not to be confused with tight and loose. In fact, inmany cases a pump with a tight performance band gives itthe ability to handle a wide flow performance range. Thewidth of the pump’s performance band describes the range of speeds in which the pump can produce acceptable flowfor the application. This is also sometimes referred to as theeffective turn-down ratio of the pump, borrowed fromterminology used in conjunction with motors or variablespeeddrives.In Figure 8, notice the point at which the green or yellowpump curves are at greatest slip point and cross the zero/noflow (x axis line). These are points (A) and (B) respectively.These are points in which the green and yellow bands,representing respective pumps, begin to produce flow underthe greatest slip condition possible for the process. Thepump that starts to produce flow at point (A) will use thetotal range of pump speed more effectively (revolutions perminute) than the pump starting at point (B).The performance band width of a pump is also affected bythe ability to drive the pump at low to high speeds. Torquerequirement, gear reduction, motor cooling and variablespeeddrive capabilities all play a part and are not in thescope of this article. Motor and variable-speed drivecapabilities, for example, set lower and upper limits.For an actual illustration of performance band width, referback to Figure 6. Notice that a pump, in this case a typicallobe pump with a 0.153 gallon/revolution theoreticaldisplacement, effectively has a narrow performanceenvelope. That is because under an arbitrary worstcondition—in this case pumping 1 cP (water-like viscosity)fluid against 75 psig—the pump only begins to produceflow at 185 rpm. This means that speeds between 50 rpm to185 rpm, which are considered good speeds for ensuring thelong life of rotary PD pumps, are not available to thepumping process. The performance band is therefore narrowas it ranges from 185 rpm (instead of 0 rpm) to themaximum mechanical speed capability of the pump, orsome other process limitation like NPSHr versus NPSHa, orthe abrasiveness of product.In comparison, refer back to Figure 7, which shows theactual performance graph of a pump with a wide performanceenvelope. Notice that under the same conditions as Figure 6—pumping 1 cP product against 75 psig—flow begins to beproduced at 15 rpm (instead of 185 rpm). In this case, onthe low-RPM range, the pump in Figure 7 produces flow at amuch wider range of RPMs than the pump in Figure 6.The lobe pump curve shown in Figure 6 does not showhow performance degrades as the pump wears. It is only a“snapshot” of the pump performance when it is new. This isthe case with most PD pumps. If wear occurs in this pump,the manufacturer-supplied performance curve no longerapplies and actual performance is unknown, unless verifiedin the field. In Figure 6, the point at which the pump beginsto produce flow under wear conditions could be evengreater than 185 rpm and prompt repairs.In sharp contrast, the pump illustrated in Figure 7compensates for wear by maintaining as-new clearances.Therefore, slip does not change, and the pump performanceremains tight with a wide range of flow capabilities. Boththe Blackmer® sliding vane pumps and Mouvex® eccentricdisc pumps share this phenomenon.Our example application that exploits these needs—thecontinuous in-line blending process—benefits from pumpsthat have a high turn-down ratio. This is because the recipeto produce the final product can be highly variable as far asthe content percentage of each ingredient. In other words,the wider the flow rate range that is achieved by the pump,the wider the variation of recipes that can be produced withthe system.

ConclusionGood flow control from rotary PD pumps offers options formore advanced processes, like in-line blending, that canhave far-reaching influence on a production facility’s overallcapital and operating costs. Respected pump manufacturersoffer performance curves that can be evaluated to determineif the performance band is comfortably suitable for theapplication. If not, alternative pumping technologies shouldbe studied and considered.Most curves do not show the effects of wear on performance.Therefore, if wear is anticipated during the expected life spanof the pumps and their parts, more subjective analysis is needed.Some curves do model wear, so look for those. Even better,some pump technologies, such as Blackmer® sliding vanepumps and Mouvex® eccentric disc technology, compensateby eliminating clearances caused by wear. Therefore, determineif these pumps are applicable for the application.Table 1 is a guide that compares different rotary PD pumptechnologies and how they compare regarding theirperformance bands and other criteria that may be important.Basically, the most important criteria for the process shouldbe heavily weighted, but none of the criteria cause adisqualification.In-line blending systems are already common in thebeverage industry where the variation of ingredientviscosities can be controlled. Several suppliers specialize inthese processes. Finding examples of more complex in-lineblending processes that demand pumps with tight and wideperformance bands is more elusive since they have beendeveloped under proprietary restrictions and confidentiality.After all, these systems, when successful, give a clearadvantage to the processor, one which they rightfully desireto keep and exploit.